1. Field of the Invention
This invention relates to a connector for mating with a printed circuit board edge card.
2. Summary of the Prior Art
In applications such as SIMM (Single In-Line Memory Module) and DIMM (Dual In-Line Memory Module) cards for computer systems, memory chips are positioned on a printed circuit and interconnected by circuit traces thereon to a plurality of juxtaposed circuit pads arranged along the edge of the card which can then be plugged into a connector for interconnecting the memory modules to a computer. In many applications, the card must be unpluggable in order to exchange or replace the card. The contact pads arranged along the edge of the card are usually provided on either side of the card. In SIMM cards, aligned contact pads on opposite sides of the board are electrically interconnected. The complementary edge card connector typically has contacts for contacting the contact pads on both sides of the board. An example is shown in U.S. Pat. No. 4,575,172, where each contact has a pair of contact points for contacting opposed aligned contact pads of the PCB. One of the contact points is thus redundant.
The contact pads of most SIMM cards are of tin or similar materials that may oxidize, the complementary contacts of the mating connector also being of a similar material. Due to the presence of oxidation layers that may impair electrical conductivity at the contact surfaces relatively high contact forces are required, and if possible a certain amount of rub during plugging connection in order to break through the oxide layers is desired. High contact forces and large numbers of contacts eliminates the practicability of having a simple plugging connection, which has led to the design of low insertion force systems such as the pivoting board solution as shown in U.S. Pat. No. 4,575,172. The pivoting lever arm effect enables high contact forces for a large number of contacts to be provided. In tin connection surfaces, for example typically used for SIMM card connectors, the requirement for high contact forces in tight spacing means that the contacts are relatively rigid and have a small elastic range. Redundant contacts are important in view of this, because warping or thickness tolerances of a mating PCB may be excessive for a single contact.
In order to generate high forces in tight spacing, SIMM connectors often have contacts edge stamped from sheet metal, where the contact spring beams flex in the plane of the sheet metal. Such contacts are usually individually assembled to connector housings by stitching, which requires a relatively high manufacturing cycle time. There are many other types of low or zero insertion force connection systems, for example some of them hold the opposed contact points apart to enable insertion of the edge card therebetween, subsequently enabling biasing together of the contacts against the contact pads by actuation of a camming mechanism or similar means. Such cammed low insertion or zero insertion force systems are often used with gold plated contact surfaces because much less contact pressure, and no rubbing effect is required between gold contact surfaces. Due to the lower contact force, edge card connector systems with gold plated surfaces are sometimes simply plugged without reducing the insertion forces because the contact forces are sufficiently low to enable this. In a given space, elastic range (flexibility) can also be increased due to the lesser requirement for contact force. Gold plating however, increases the cost of the connection system.
It would be desirable to reduce the cost of such connection systems whilst nevertheless maintaining the requisite level of reliability and performance.